The note mentioned "domestic problems", Police Lieutenant Somyot Booyakaew said.
"The letter was written by Karl Slym's wife about domestic problems," Somyot said, adding that Slym's death was not a murder.
51-year-old Karl Slym's body was found on Sunday by the staff of the riverside Shangri-la Hotel on the balcony of the fourth floor after plunging from the 22nd floor.
He was here to attend a meeting of the Board of Directors of the Thai subsidiary of his company.
Somyot earlier said there were no signs of a struggle in the British national's room, which did not have a balcony but had a big sealed glass window and another smaller window that could be opened.
The room has a big sealed window overlooking the Chao Praya River and a smaller window which could be opened nearby.
The police said yesterday that they were translating the note into Thai language as part of the investigation.
Sally, Karl Slym's wife of 30 years, was with him in the room and was woken by hotel staff on Sunday morning after they found his body.
Karl Slym was leading Tata Motors at a time when the auto industry was grappling with a prolonged slowdown.
He joined the auto major in 2012 as part of a major management overhaul and was responsible for charting the company's strategy to regain momentum in the Indian market.